Oxygen concentrators have, since their popularity began in the 1970s, been considered a most cost effective and preferred delivery system for the supply of supplemental oxygen therapy in homes and extended care facilities to many types of patients, such as patients with chronic obstructive pulmonary disease. These concentrators can, in a one step real-time process, provide oxygen at levels of 90 to 96 percent concentration, which has been found to be substantially the clinical equivalent to USP oxygen at 99.5% purity that is available for such uses from high pressure cylinders or liquid oxygen systems. Most commercially available oxygen concentrators separate and concentrate the oxygen from ambient air to the 90 to 96% concentration level by the technique of pressure swing adsorption or PSA.
Pressure swing adsorption is a process for separating gasses from gas mixtures, and is now a process well known for its ability of producing concentrated oxygen from air. In pressure swing adsorption processes, the gas mixture, such as air, is pumped into a sieve bed that is formed of a gas tight container filled with a granulated or particulate molecular sieve material, such as zeolite, of a special grade commercially available for such process. For the separation of oxygen from air, 5A-zeolite(zeolite of five angstrom unit pore size) is one such molecular sieve material that is commonly used. In the separation process, the sieve material selectively attracts and adsorbs gasses from the mixture. In a process for separating oxygen from air, gasses such as carbon dioxide and water vapor are first adsorbed by the particles within the sieve material. More importantly, the next gas adsorbed by the sieve bed material is nitrogen, which is adsorbed by the sieve material from the air in preference to oxygen. As a result, a properly sized, pressurized and cycled sieve bed will remove essentially all of the gasses from the air leaving a mixture of only oxygen and argon. The argon will not be adsorbed by the sieve material before the adsorption of oxygen. The resulting product gas has a purity of in excess of 90% oxygen, and in many systems can have a concentration of approximately 96% oxygen and 4% argon. Molecular sieve beds are, for simplicity, referred to below as sieves.
The typical oxygen concentrator that uses PSA to produce oxygen for oxygen therapy uses two sieves of at least 1500 cm.sup.3 to produce concentrated oxygen at a rate of at least 2 liters per minute (LPM). Operated through a cycle in which the sieves are alternatively pressurized for 30 seconds to 30 psig (30 pounds per square inch above atmospheric pressure), and then depressurized for 30 seconds to atmospheric pressure, the sieves each produce one liter of product gas per cycle, or two LPM. The oxygen concentrator systems of the prior art have, however, been larger and heaver than is desirable, have consumed more energy in the operation of the pump than is desirable, and have generated noise and vibration. When the flow rates are increased or the pressures varied in an effort to reduce the size of the concentrator, it has been found that the concentration of oxygen in the product gas quickly declines as nitrogen breaks through the bed and enters the effluent product gas stream.
Accordingly, there remains a need for a superior oxygen concentrator that is light in weight and compact in size, that is quiet, that utilizes energy efficiently and that produces oxygen at 2 to 5 LPM in concentrations of at least 90% and preferably 96%.